Embodiments of the present invention relate to integrated circuits and the processing for the manufacture of semiconductor devices. More particularly, embodiments of the present invention provide structures of an embedded microlens that can be made and packaged easily and methods for making such structures. The microlens according to embodiments of the present invention can be applied to charge-coupled devices (CCDs), color CMOS image sensors, contact image sensors, and others. But it would be recognized that the invention has a much broader range of applicability. For example, the invention can be applied to make a variety of photometric devices and distance measuring devices containing microlens to increase the amount of light impinging on a photo-sensing element and to improve its sensitivity.
Integrated circuits or “ICs” have evolved from a handful of interconnected devices fabricated on a single chip of silicon to millions of devices. Current ICs provide performance and complexity far beyond what was originally imagined. In order to achieve improvements in complexity and circuit density (i.e., the number of devices capable of being packed onto a given chip area), the size of the smallest device feature, also known as the device “geometry”, has become smaller with each generation of ICs. Semiconductor devices are now being fabricated with features less than a quarter of a micron across.
Increasing circuit density has not only improved the complexity and performance of ICs but has also provided lower cost parts to the consumer. An IC fabrication facility can cost hundreds of millions, or even billions, of dollars. Each fabrication facility will have a certain throughput of wafers, and each wafer will have a certain number of ICs on it. Therefore, by making the individual devices of an IC smaller, more devices may be fabricated on each wafer, thus increasing the output of the fabrication facility. Making devices smaller is very challenging, as each process used in IC fabrication has a limit. That is to say, a given process typically only works down to a certain feature size, and then either the process or the device layout needs to be changed. An example of such a limit in photo-electronic IC fabrication, such as image sensors, is the ability to make microlenses properly to increase the amount of light impinging on the photo-sensing pixel as the pixel side is decreased for achieving better image resolution.
As merely an example, a conventional microlens fabrication process is often performed after the silicon processing and often performed in conjunction with a color filter coating. After the formation of R, G, B color filters, a layer of planarization coating is often applied before a microlens coating material is applied thereon. Subsequently, photolithography and thermal curing processes are performed to form the microlens. This conventional approach, wherein microlenses are formed on top of the color filters, requires separate material and lithography processing, and non-standard packaging methods due to the existence of an air gap for facilitating proper focusing of incident light beams. Additionally, the microlenses, which typically include resin, are formed at relatively low temperature (about 200° C.). The low temperature process may limit the use of the microlenses in high-temperature soldering applications.
From the above, it is seen that improved techniques for forming microlenses are desired.